This invention relates to an improved method for preparing a cyclic siloxane of the general formula: EQU [(CH.sub.3).sub.2 SiO].sub.5 I
(also known as "pentamer" or D.sub.5) from octamethyl cyclotetrasiloxane of the formula EQU [(CH.sub.3).sub.2 SiO].sub.4
(also known as "tetramer" or D.sub.4) by treating the tetramer with a normal C.sub.6-16 alkyl sulfonic acid having the formula EQU n-C.sub.x H.sub.2x+1) SO.sub.3 H II
where x=6 to 16, inclusive
in the presence of aqueous HCl at elevated temperatures.
The term "normal C.sub.6-16 alkylsulfonic acid" is intended to include not only the acid itself but also its negatively charged alkali metal salts thereof, for instance, the sodium salt of the sulfonic acid. For convenience, availability, and cost, the sodium salt (which is an anionic surfactant while the corresponding free sulfonic acids are nonionic surfactants) is used in the examples which are illustrative of this invention. However, essentially all the sodium salts are analogues of the sulfonic acids and are converted to the acid in the presence of the aqueous HCl. Therefore, the active catalyst is the alkyl sulfonic acid, or mixtures thereof.
Among the n-alkyl sulfonic acids which can be employed in the practice of the present invention, in addition to those recited in the following examples, may be mentioned for instance, C.sub.8 H.sub.17 SO.sub.3 H, C.sub.10 H.sub.21 SO.sub.3 H, C.sub.12 H.sub.25 SO.sub.3 H, C.sub.14 H.sub.29 SO.sub.3 H, C.sub.16 H.sub.33 SO.sub.3 H, etc. Included within the scope of these sulfonic acids are the precursor alkali-metal salts thereof, such as the sodium, potassium, etc. salts, which under the reaction conditions of carrying out the invention, are converted to the sulfonic acid.